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A return vent downstairs and one return upstairs - but they are connected in the same run to the system, so possibly that means it’s just a central return?

Every room has a single ceiling vent, except for the downstairs living room, there are two vents that are about 6 ft apart.

The vent sizes seem to be about 6 in (based upon the size of the grates) except it one room - it appears to be 8 in. It’s a larger grate than others.

The attic looks to be properly insulted based upon the depth of cover of the insulation. We just had the roof replace and the vents replaced. We sealed the gap around the attic drop down stairs to close that gap.

I am just at a loss - this seems to be a common design I our neighborhood and we have a newer outside unit that is larger than our two neighbors and they don’t complain about the same issues.

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ok...

here is what you need to do.

walk around your house.

look at the roof design.

note if it has metal turbines, or power fans, or a ridge, or gable grills on the ends of each peak...

note if the overhang is vented or not. (it's called a soffit)

unfortunately, few if any roofers understand how a attic should be vented... most say... ridge vent works best, and gable being open helps too. soffit isn't needed... all these things are incorrect.

best is a radiant barrier, with a foam insulated roof deck, and a totally sealed attic.

second is a radiant barrier, with a web supported blown in insulation to the roof deck, and sealed soffit.

third is the ridge vent, IF the gable is sealed, AND the soffits are open front and back, AND all insulation in the attic floor is removed, all gaps and penetrations fully sealed, then R40 insulation put down on top.

all else is a poor attempt to ventilate the attic.

the thing is, it is near impossible to repair poor ductwork that is in the lower floor, between 1st and 2nd floor... without removing the wallboard... the best you can do is get the top floor attic ducts sized to handle the CFM requirements of the floor, and hope for the best with the rest.

sizing the system to the LOAD of the house is far better than putting in a bigger system, on INADEQUATE ductwork...